Field
Example embodiments relate to a method of enabling a plurality of user equipments (UEs) to transmit on a random access channel (RACH), an access point (AP) configured to perform the same, a method of transmitting an access request on the RACH from a UE and a UE configured to perform the same.
Related Art
Conventionally, random access channels (RACHs) or propagation paths are used by one or more user equipments (UEs) to initiate access to a Universal Mobile Telecommunications System (UMTS) network.
In conventional UMTS networks, RACHs are transport channels, which carry data mapped from upper level logical channels (e.g., Open Systems Interconnect (OSI) Layers 3-7).
A requesting UE requests access to the UMTS network, using the RACH by transmitting a random access channel preamble to a serving access point (AP) at a desired power level. An initial transmission power level for the random access channel preamble is determined by the requesting UE, for example, using a measured pilot power in the downlink from the serving AP to the requesting UE.
The RACH is characterized by a fixed set of time and frequency resources. For a given transmission time interval, the frequency resources of the RACH are typically partitioned into orthogonal resources (for example, subbands or spreading codes). To communicate over the RACH, a UE transmits over one of these orthogonal frequency resource (for example, by randomly selecting one of the orthogonal frequency resources).
After transmitting an initial random access channel preamble to request access to the wireless network, the UE waits for an acknowledgement (ACK) or a negative acknowledgement (NACK) from the AP. If an acknowledgement is received within a time period, the UE transmits a subsequent data message. Alternatively, if the UE does not receive an acknowledgement, the UE transmits another random access channel preamble having an increased transmission power level, and in turn an increased energy value. The UE may repeat this procedure until an acknowledgement is received from the AP or the UE reaches a maximum allowed number of random access channel preambles transmitted in one access attempt.
The AP may fail to receive the random access channel preamble because the signal-to interference-plus noise ratio (SINR) is insufficient to detect the preamble. The SINR may be insufficient because, irrespective of the increased transmission power level, the signal strength of the preamble is too weak (e.g., the UE is located too far away from the AP). Further, the noise (e.g., interference) from other UEs within the cell may be interfering with the transmission of the preamble by the requesting UE. This interference would occur, for example, if the requesting UE and interfering UE transmit simultaneously on the same orthogonal frequency resource.
As the number of UEs in a cell increase, the probability of multiple UEs selecting the same frequency resource increases, and in this case the preambles transmitted by the multiple UEs may be indistinguishable at the AP.